The present invention relates to a needle thread holding device in a sewing machine provided with a thread cutting unit.
According to a conventional sewing machine having a thread cutting unit, a thread drawn from the lastly stitched point of a workpiece or a fabric is required to be cut when the sewing operation is terminated. In this case, if the needle thread length between a needle eye and the exact cutting point is insufficiently short, the needle thread may be easily disengaged from the eye of the needle at a start phase of a subsequent sewing operation. Therefore, the thread cutting must be made so that sufficient residual thread length can be provided between the needle eye and the exact cutting point in order to retain the thread to be still engaged with the thread hole at the start timing of the subsequent sewing operation. However, if the residual thread length is excessively long, the residual thread may be stitched in an entangled manner at the next sewing operation in a nest fashion, to thereby degrade stitching quality.
To avoid these drawbacks, a thread holding device which is adapted to hold the needle thread has been provided so as to provide optimum length of the residual needle thread at the time of thread cutting. That is, the thread holding device holds the cut end portion of the thread after the thread cutting operation in order to provide sufficiently small residual length of the thread yet preventing the thread from being disengaged from the eye of the needle in the next sewing operation. However, in the conventional thread holding device, excessively high tension may be imparted on the needle thread during a time period in which the needle thread is released from the thread holding device thereby overcoming the thread retaining force thereof in accordance with the feeding of the workpiece after the start of the subsequent sewing operation. Accordingly, a stitch balancing thread tension may be disorganized, and a stitched workpiece may be shrunk.
In another aspect, at a start timing of the sewing operation, a presser foot located at its non-operative position is moved downwardly to its operative position for depressing the workpiece onto a workpiece supporting surface of the sewing machine. However, in order to perform prompt sewing operations, prior to the depression of the workpiece by the pressure foot, the sewing machine may have been energized for starting actual stitching. In this case, however, the workpiece is not stably held. Therefore, a knot between the needle thread and a bobbin thread is not provided to cause a skip stitching. In a conventional sewing machine having an automatic pressure bar lifter, the sewing machine is energized after the workpiece is stabilizingly held by the pressure foot. However, such sequence does not meet with the prompt sewing work, and an operator may be dissatisfied with such dull sewing sequence if compared with the conventional rhythmical sewing work, i.e., sewing start prior to the complete depression of the workpiece by the pressure foot.